Fuel cell assembly

ABSTRACT

In a fuel cell assembly typically with a plurality of cells each including an electrolyte layer ( 2 ), a pair of gas diffusion electrode layers ( 3, 4 ), and a pair of flow distribution plates ( 5 ), the electrolyte layer ( 2 ) comprises a frame ( 21 ) and electrolyte ( 22 ) retained in the frame; and the flow distribution plates and frames are made of materials having similar thermal expansion properties so that the generation of thermal stress between the frames of the electrolyte layers and the corresponding flow distribution plates can be avoided, and the durability of the various components can be ensured. By joining each flow distribution plate with the corresponding frame by anodic bonding or using a bonding agent along a periphery thereof, the need for a sealing arrangement such as a gasket or a clamping arrangement can be eliminated, and this contributes to the compact design of the assembly.

This application claims the benefit of U.S. Provisional Application Nos.60/202,827, filed May 8, 2000, and 60/242,136, filed Oct. 23, 2000, bothof which are herein incorporated by reference.

TECHNICAL FIELD

The present invention relates to a fuel cell assembly typically with aplurality of fuel cells each including an electrolyte layer, a pair ofgas diffusion electrode layers placed on either side of the electrolytelayer, and a pair of flow distribution plates placed on either outerside of the gas diffusion electrode layers to define passages fordistributing fuel gas and oxidizing gas in cooperation with the opposingsurfaces of the gas diffusion electrode layers.

BACKGROUND OF THE INVENTION

A fuel cell includes an electrolyte layer and a pair of electrodesplaced on either side of the electrolyte layer, and generateselectricity through an electrochemical reaction between fuel gas such ashydrogen and alcohol and oxidizing gas such as oxygen and air, which aresupplied to the corresponding electrodes, with the aid of a catalyst.Depending on the electrolytic material used for the electrolyte layer,the fuel cell may be called as the phosphoric acid type, solid polymertype or molten carbonate type.

In particular, the solid polymer electrolyte (SPE) type fuel cell usingan ion-exchange resin membrane for the electrolyte layer is consideredto be highly promising because of the possibility of compact design, lowoperating temperature (100° C. or lower), and high efficiency.

The SPE typically includes an ion-exchange resin membrane made ofperfluorocarbonsulfonic acid (Nafion: tradename), phenolsulfonic acid,polyethylenesulfonic acid, polytrifluorosulfonic acid, and so on. Aporous carbon sheet impregnated with a catalyst such as platinum powderis placed on each side of the ion-exchange resin membrane to serve 5 asa gas diffusion electrode layer. This assembly is called as amembrane-electrode assembly (MEA). A fuel cell can be formed by defininga fuel gas passage on one side of the MEA and an oxidizing gas passageon the other side of the MEA by using flow distribution plates(separators).

Typically, such fuel cells are stacked, and the flow distribution platesare shared by the adjacent fuel cells in the same stack. When formingsuch a stack, it is necessary to seal off the passages defined on thesurfaces of the MEAs from outside. Conventionally, gaskets were placedin the periphery of the interface between each adjoining pair of a MEAand a distribution plate. The contact area between the MEA and the gasdiffusion electrode was ensured by pressing them together by applying anexternal force, typically with the aid of a suitable fastener. Therequired electric connection between the gas diffusion electrode and anelectrode terminal connected to an external circuit was also ensured bypressing them together by applying an external force.

However, because the MEA changes its volume depending on the watercontent and temperature of the SPE, the external force applied by afastener inevitably changes, and this may impair the sealing capabilityof the assembly. The SPE may be surrounded by a frame to stabilize theshape of the SPE, but because the frame and flow distribution platesthermally expand and contract individually, the external force appliedby the fastener still changes. The change in the external force in thiscase produces stresses in the various members, and this may impair thedurability of the various members of the assembly.

The packaging and/or the arrangement for ensuring such a controlledpressure and a required sealing performance tends to be large in size,and this has prevented a compact design for the fuel cell assembly.Furthermore, even with a highly elaborate arrangement for ensuring asealing performance, due to the uneven thermal expansion and contractionof various parts, it has been difficult to maintain the required sealingperformance for an extended period of time.

BRIEF SUMMARY OF THE INVENTION

In view of such problems of the prior art, a primary object of thepresent invention is to provide a fuel cell assembly which can ensure afavorable seal under all conditions.

A second object of the present invention is to provide a fuel cellassembly which can ensure a reliable electric contact between theelectrode terminal and the gas diffusion electrode.

A third object of the present invention is to provide a fuel cellassembly which is highly compact and efficient at the same time.

A fourth object of the present invention is to provide a fuel cellassembly which is easy to manufacture.

According to the present invention, such objects can be accomplished byproviding a fuel cell assembly with at least one cell each including anelectrolyte layer, a pair of gas diffusion electrode layers interposingthe electrolyte layer between them, and a pair of flow distributionplates for defining passages for fuel and oxidizer gases that contactthe gas diffusion electrode layers, so that: the electrolyte layercomprises a frame and electrolyte retained in the frame; and each flowdistribution plate and the corresponding frame are joined along aperiphery thereof so as to achieve an air-tight cavity between them.Each flow distribution plate and the corresponding frame may be joinedby anodic bonding, diffusion bonding, welding, brazing, and using abonding agent.

This allows the flow distribution plates and frames to be joined with arequired sealing capability without requiring any clamping arrangement,fasteners or any other arrangements relying on an external force.

In particular, if the flow distribution plates and frames are made ofmaterials having similar thermal expansion properties, even when theassembly is subjected to changes in temperature, because the frame forthe electrolyte layer and the flow distribution plates expand andcontract in a similar manner, creation of any internal stress in theassembly can be avoided. This contributes to the conservation of thesealing performance of the assembly and enhancement of the durability ofthe assembly over an extended period of time and even when subjected toextreme conditions.

Preferably, the flow distribution plates and the frame for theelectrolyte layer may be both made of a silicon wafer which is suitedfor micro working processes. According to a preferred embodiment of thepresent invention, the flow distribution plates and the frame for theelectrolyte layer are joined along their periphery by using anodicbonding or a bonding agent. This eliminates the need for any gaskets orclamping arrangements, and simplifies the structure of the assemblywhile increasing the reliability of the assembly over an extended periodof time and under extreme conditions.

BRIEF DESCRIPTION OF THE DRAWINGS

Now the present invention is described in the following with referenceto the appended drawings, in which:

FIG. 1 is an exploded perspective view of a part of a fuel cell assemblyembodying the present invention;

FIG. 2 a is a sectional view taken along line IIa—IIa of FIG. 1;

FIG. 2 b is a sectional view taken along line IIb—IIb of FIG. 1;

FIGS. 3 a to 3 c are sectional views of the electrolyte layer indifferent steps of the fabrication process; and

FIGS. 4 a to 4 c are sectional views of the flow distribution plate indifferent steps of the fabrication process.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows the structure of a part of a fuel cell assembly embodyingthe present invention. In practice a plurality of cells are formed intoa stack, and a number of such stacks are connected in series and/orparallel, and fuel such as reformed alcohol, hydrogen gas or the like issupplied to each fuel cell stack along with oxidizing gas such as air.

Referring also to FIGS. 2 a and 2 b, each fuel cell 1 includes a centralelectrolyte layer 2, a pair of gas diffusion electrode layers 3 and 4(see FIGS. 2 a and 2 b) placed on either side of the central electrolytelayer 2, and a pair of flow distribution plates 5 placed on either outerside of the gas diffusion electrode layers 3 and 4. The outer side ofeach flow distribution plate 5 is similarly formed as the inner sidethereof so as to serve as the flow distribution plate for the adjacentfuel cell.

The electrolyte layer 2 comprises a grid frame 21, and solid polymerelectrolyte (SPE) 22 which is filled into rectangular through holes 21 bdefined between adjacent bars 21 a of the grid frame 21. The SPE 22 maybe made from such materials as perfluorocarbonsulfonic acid (Nafion:tradename), phenolsulfonic acid, polyethylenesulfonic acid,polytrifluorosulfonic acid, and so on.

The grid frame 21 is formed by etching or otherwise working a siliconwafer, and is provided with a rectangular and annular fringe portion anda rectangular grid area defined inside the annular fringe portion. Eachbar 21 a in the grid area of the grid frame 21 is provided with aprojection 21 c at an intermediate part thereof so as to project into anintermediate part of the corresponding through hole 21 b as best shownin FIGS. 2 a and 2 b. The projection 21 c is in the shape of a ridgeextending along the length of the bar 21 a, and produces a narrowermiddle part in each through hole 21 b. The projection 21 c helps toretain the SPE 22 in each through hole 21 b.

Such a projection can be conveniently formed at the same time as formingthe grid frame 21. FIGS. 3 a to 3 c illustrate the process of formingthe electrolyte layer 2. First of all, a suitably patterned photoresistlayer 13 and 14 is placed on each side of a silicon wafer serving as thematerial for the grid frame 21 as shown in FIG. 3 a. An anisotropicetching is performed from both sides of the wafer as illustrated in FIG.3 b, and this produces a plurality of through holes 21 b each of whichis narrowed in a middle part by a projection 21 c. Then, SPE 22 isfilled into each of the through holes 21 b so as to define asubstantially flush planar surface on each side of the electrolyte layer2.

In this embodiment, a rectangular through hole 23 a, 23 b, 24 a and 24 bis formed in each corner portion of the fringe portion of the grid frame21. One of the diagonally opposing pairs of these through holes 23 a and23 b serve as inlet and outlet for the fuel gas. The remaining opposingpair of these through holes 24 a and 24 b serve as inlet and outlet forthe oxidizing gas.

Each flow distribution plate 5 is also formed by working a siliconwafer, and has a substantially conformal rectangular shape. Arectangular recess 51 or 52 having a flat bottom is formed centrally oneach side of the flow distribution plate 5, and a plurality ofprojections 53 or 54 each having the shape of a truncated pyramid areformed on the flat bottom. The surface of the recesses and theprojections are coated with a gold plate layer serving as an electrodeterminal layer 55 or 56 by suitable means for electrically connectingthe gas diffusion electrode layers 3 and 4 to an external circuit.

FIGS. 4 a to 4 c show the process of forming each flow distributionplate 5. A suitably patterned photoresist layer 15 and 16 is formed oneach side of a silicon wafer as shown in FIG. 4 a, and the silicon waferis etched from both sides to form the recesses 51 and 52 and projections53 and 54 at the same time as shown in FIG. 4 b. The distribution plate5 on the upper end or lower end of each fuel cell stack may be providedwith a recess and projections only on inner side thereof. Thereafter,electrode terminal layer 55 and 56 is formed over the surface of therecesses 51 and 52 and projections 53 and 54 as shown in FIG. 4 c.

The distribution plate 5 is conformal to the grid frame 21, andtherefore has a rectangular shape. A rectangular through hole 57 a, 57b, 58 a or 58 b is formed in each corner portion of the fringe portionthereof. One of the diagonally opposing pairs of these through holes 57a and 57 b serve as inlet and outlet for the fuel gas. The remainingopposing pair of these through holes 58 a and 58 b serve as inlet andoutlet for the oxidizing gas. As shown in FIG. 1, grooves 59 a and 59 bformed in the fringe portion communicate the recess 51 with the throughholes 58 a and 58 b for the oxidizing gas, and similar grooves 60 a and60 b communicate the recess 52 with the through holes 57 a and 57 b forthe fuel gas.

The gas diffusion electrode layers 3 and 4 each include a carbon sheet 3a or 4 a having a layer of a platinum catalyst 3 b and 4 b mixed withSPE formed on the side thereof facing the electrolyte layer 2.

In this manner, in each fuel cell, a pair of flow distribution plates 5are placed on either side of an electrolyte layer 2 via a gas diffusionelectrode layer 3 or 4, and these components are joined by anodicbonding along the parts surrounding the recesses. Therefore, a pluralityof narrow passages 11 are defined in one of the central recesses 52 ofeach electrolyte layer 2 for the fuel gas, and a plurality of similarnarrow passages 10 are defined in the other of the central recesses 51of the electrolyte layer 2 for the oxidizing gas. Each projection issubstantially entirely covered by a gold plate layer serving as anelectrode terminal, and lightly pushes the gas diffusion electrode layer3 or 4 against the frame grid 21 of the electrolyte layer 2. Therefore,each gas diffusion electrode layer 3 or 4 is electrically connected tothe corresponding distribution plate 5 via a large number of projectionsin a parallel relationship, and a reliable electric connection betweenthe electrolyte layer 2 and an external circuit can be established.

The adhesion between the grid frame 21 and the distribution plates 5 canbe accomplished in a number of different ways. Preferably, anodicbonding is used as described in the following. An electrode layer 9 anda layer 8 of heat resistance and hard glass, for instance, made of Pyrexglass (tradename) are formed along the peripheral surface of the gridframe 21 of the electrolyte layer 2 on each side thereof by sputtering,and a similar electrode layer 9 is formed along the peripheral part ofthe opposing surface of the distribution plates 5. Then, with thisassembly heated to about 400° C. at which sodium ions become highlymobile, an electric field is produced in the assembly so as to moveions. In the fuel cell assembly of the present invention, if theelectrolyte includes a solid polymer, heating the entire assembly to thetemperature of 400° C. may damage the solid electrolyte. Therefore,according to this embodiment, a heater (not shown) is placed under theelectrode layer 9 to selectively heat only the peripheral part of theflow distribution plates. The heater may include polycrystalline siliconsandwiched between insulating layers such as Si₃N₄ layers. If theelectrode terminal layer 55 and 56 extend under the heater, the thermalefficiency of the heater will be impaired. Therefore, it is preferableto omit the electrode terminal layer 55 and 56 from under the heater.

The grid frame 21 and the distribution plates 5 are placed one overanother, and compressed at a pressure of 100 gf/cm² to 2,000 gf/cm².Electric current is conducted through the polycrystalline silicon heaterto locally heat the bonded area to a temperature in the other of 400° C.At the same time, a voltage in the order of 100 to 500 V is appliedbetween the electrode layer 9 of the grid frame 21 and the electrodelayer 9 of the distribution plate 5 for 10 to 30 minutes.

Alternatively, a bonding agent may be used for attaching the grid frame21 and the distribution plates 5 together. In either case, it ispossible to eliminate the need for any sealing arrangements or clampingarrangements to achieve a desired sealing capability, and this allows acompact design of the fuel cell assembly.

As the fuel gas and oxidizing gas (air) are conducted through this fuelcell 1, an electrochemical reaction takes places by virtue of theplatinum catalyst, and an electric voltage develops between theelectrode terminal layers 55 and 56. A number of such fuel cells arestacked so that a desired voltage can be obtained.

Although the fuel and oxidant for the fuel cells described hereinconsists of gases, they may also include liquids.

Although the present invention has been described in terms of preferredembodiments thereof, it is obvious to a person skilled in the art thatvarious alterations and modifications are possible without departingfrom the scope of the present invention which is set forth in theappended claims.

1. A fuel cell assembly comprising at least one cell comprising anelectrolyte layer (2), a pair of gas diffusion electrode layers (3, 4)interposing the electrolyte layer between them, and a pair of flowdistribution plates (5) for defining passages (10, 11) for fuel andoxidizer gases that contact the gas diffusion electrode layers,characterized by that: the electrolyte layer (2) comprises a frame (21)and electrolyte (22) retained in the frame; the flow distribution platesand the frame are made of materials having similar thermal expansionproperties; and wherein each flow distribution plate and thecorresponding frame are joined along a periphery thereof so as toachieve an air-tight cavity between them.
 2. A fuel cell assemblyaccording to claim 1, wherein each flow distribution plate and thecorresponding frame are joined by anodic bonding along a peripherythereof so as to achieve an air-tight cavity between them.
 3. A fuelcell assembly according to claim 1, wherein each flow distribution plateand the corresponding frame are joined by a bonding agent along aperiphery thereof so as to achieve an air-tight cavity between them. 4.A fuel cell assembly comprising at least one cell comprising anelectrolyte layer (2), a pair of gas diffusion electrode layers (3, 4)interposing the electrolyte layer between them, and a pair of flowdistribution plates (5) for defining passages (10, 11) for fuel andoxidizer gases that contact the gas diffusion electrode layers,characterized by that: the electrolyte layer (2) comprises a frame (21)and electrolyte (22) retained in the frame; and wherein the flowdistribution plates and the frame are made of silicon substrates havingsimilar thermal expansion properties.